1. Technical Field
The present invention relates generally to field effect transistors (FETs), and in particular, to such transistors formed in wide bandgap semiconductor materials. Further, this invention relates to monolithic and hybrid integrated circuits comprising low-voltage control circuitry and to power switches built using the above transistors.
2. Background of the Technology
Wide bandgap semiconductor materials (with EG>2 eV) such as silicon carbide (SiC) or Group III nitride compound semiconductors (e.g., gallium nitride or GaN) are very attractive for use in high-power, high-temperature, and/or radiation resistant electronics. Monolithic or hybrid integration of a power transistor and control circuitry in a single or multi-chip wide bandgap power semiconductor module is highly desirable for such applications in order to improve the efficiency and reliability of the system.
SiC smart power technology has been a topic of discussion in recent years, but has experienced limited scientific investigation. Proposed solutions have been met with skepticism relating to the operation of both the power switch and control circuitry.
Because of the fundamental differences in material properties and processing technologies, traditional Si or GaAs integrated circuit (IC) technologies such as Complementary Metal-Oxide-Semiconductor (CMOS) or Direct Coupled FET Logic (DCFL) cannot in most cases be easily transferred to wide bandgap semiconductors. Several attempts at fabricating SiC NMOS and CMOS digital and analog ICs have been reported in the last decade (e.g., [1], [2]). A monolithic CMOS integrated device in SiC and method of fabricating the same is disclosed in U.S. Pat. No. 6,344,663, [3]. Moreover, recent development in SiC Lateral DMOS Field-Effect Transistors (LDMOSFETs) (e.g., [4]–[5]) theoretically allow for the monolithic integration of MOSFET-based control circuitry and power switches for use in Smart Power electronics. Various issues, however, limit the use of MOSFET-based SiC integrated circuits in the applications where high-temperature and/or radiation tolerance is required. The first such issue is on-state insulator reliability as a result of a much smaller conduction band offset of SiC to SiO2 as compared to that of silicon. This issue becomes even more significant at high temperatures and in extreme radiation environments. Other issues include: low inversion channel mobility due to high interface state density at the SiC/SiO2 interface and high fixed charge density in the insulator; and significant threshold voltage shift with temperature due to ionization of interface states.
Another transistor candidate for use in SiC Smart Power electronics, a SiC bipolar junction transistor (BJT), also suffers from interface-related issues such as high recombination velocity on the surface between the emitter and the base resulting in low current gain and high control losses.
Another transistor candidate for use in SiC Smart Power electronics is a Metal Semiconductor Field-Effect Transistor (MESFET). Despite the fact the SiC MESFET monolithic microwave integrated circuits (MMICs) received significant development in the last decade (e.g., [6]), there have been few published attempts to build SiC MESFET logic and analog circuits (e.g., [7]).
An alternative to the MOSFET and MESFET approaches is the use of lateral JFET-based integrated circuits implemented in either complementary (n-type and p-type channels as disclosed in U.S. Pat. No. 6,503,782 [8]) or enhanced-depletion (n-type channels) forms. SiC JFETs have proven to be radiation tolerant while demonstrating very insignificant threshold voltage shift with temperature. Encouraging results in the development of high-temperature normally-on power vertical junction field-effect transistors (VJFETs) have been published in recent years (e.g., [9]). However, despite their excellent current-conduction and voltage-blocking capabilities, a major deficiency of these transistors is that they are “normally-on” devices. On the system level, this often requires an additional (negative) supply voltage and short circuit protection.
Several attempts to build normally-off SiC high-voltage VJFET switches have been reported recently. Typically, these devices comprise both lateral and vertical channel regions (e.g., [10]–[12]). These devices, however, exhibit a drastic contradiction between the device blocking capabilities and the specific on-resistance. For example, a VJFET with a 75 μm, 7×1014 cm−3 n-type drift region was able to block above 5.5 kV at zero gate-to-source voltage [13]. At the same time, this device demonstrated a specific on-resistance (Rsp-on) of more then 200 mΩ*cm3. The intrinsic resistance of its drift layer estimated from its thickness and doping was slightly above 60 mΩ*cm3, with the remainder of the on-resistance was contributed by the channel regions.
In order to reduce the specific on-resistance of SiC power VJFETs, these devices can be driven in bipolar mode by applying high positive gate-to-source voltage. For example, the device discussed above and disclosed in [13] demonstrated an Rsp-on of 66.7 mΩ*cm3 when a gate-to-source bias of 5 V was applied [14]. This approach, however, can lead to significant power losses due to high gate current.
Another approach is to use special circuits and methods for controlling normally-on devices so that they can be operated in normally-off mode. A cascode connection of a low-voltage control JFET with a high-voltage JFET wherein the drain of the control JFET is connected to the source of the high-voltage device and the gate of high-voltage JFET is connected to the source of the control JFET has been disclosed in U.S. Pat. No. 3,767,946 [15]. A compound field-effect transistor monolithically implementing such a cascode connection has also been disclosed in U.S. Pat. No. 4,107,725 [16]. Similar types of cascode circuits, where low-voltage normally-off devices control high-voltage normally-on devices are disclosed in U.S. Pat. No. 4,663,547 [17]. More recently, a normally-on SiC VJFET controlled by an Si MOSFET in the above configuration has been reported by several groups (e.g., [18]). This integrated power switch has demonstrated excellent voltage-blocking and current-conducting capabilities, as well as high switching speed. However, the use of silicon MOSFETs for the control of power in normally-on SiC VJFETs significantly limits both the temperature range and the radiation tolerance of the cascode. Accordingly, there is still a need for wide bandgap normally-off power switching device in general, and in particular, for such a power switch integrated with control circuitry built in wide bandgap semiconductors.